The specific morphology of human skin as well as other biological tissues gives rise to scattering of light. Skin may be the largest organ in the human body and composed of three distinct structures, as shown, for example, in FIG. 1. From the surface downward, they may be: 1) the epidermis; 2) dermis; and 3) subcutaneous fat (not shown). The epidermis may be the thinnest structure, varying in thickness from about 40 μm on the eyelids to about 1.6 mm on the palmar surface of the hand. The average epidermal thickness may be about 100 μm. The most superficial layer of the epidermis may be a dead outer layer, the stratum corneum, which may be responsible for the skin's chemical impermeability. Together, the stratum corneum and epidermis protect the human body from a variety of insults of physical, chemical, electrical, radiologic or microbiologic origin. The remainder of the epidermis may be a metabolically active, stratified squamous, cornifying epithelium generally populated by four types of cells: keratinocytes, melanocytes, Langerhans cells and Merkel cells in descending population frequency.
Keratinocytes form the bulk of the epidermis and undergo a specific form of cellular differentiation which may create the dead, superficial layers of the skin. Melanocytes, located in the deeper layers of the epidermis (basement membrane) may be capable of producing melanin which comprises the pigmentary system of the skin. Langerhans cells may serve an immunological function related to macrophages. Merkel cells may be receptors presumed to be involved in touch perception.
The dermis may be much thicker (1-2 mm) than the epidermis and may be subdivided into two compartments: 1) a thin zone immediately below the epidermis—the papillary dermis; and 2) a thick zone that extends from the base of the papillary dermis to the subcutaneous fat—the reticular dermis. The papillary dermis may be characterized by a network of thin (0.3-3 μm diameter) collagen fibers and elastic fibers (10-12 μm diameter), embedded in loose connective tissue and a highly developed microcirculation composed of arterioles, capillaries and venules. The reticular dermis may be composed predominantly of dense bundles of thick (10-40 μm diameter) collagen fibers that may be arranged primarily parallel to the skin's surface, interspersed among which may be coarse elastic fibers and fibroblasts embedded in an amorphous ground substance material containing water, electrolytes, plasma proteins and mucopolysaccharides. The latter consist of long-chain glycosaminoglycans which retain water in amounts up to about 1000× their own volume.
The scattering properties of human skin substantially constrain the development of novel approaches for both light-based therapeutics and diagnostics.